Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger located in a wellhead housing and that supports a string of casing extending into the well. A seal or packoff seals between the casing hanger and the wellhead housing. Alternatively, the inner wellhead member could be a tubing hanger that supports a string of tubing extending into the well for the flow of production fluid. The tubing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a tubing head. A packoff or seal seals between the tubing hanger and the outer wellhead member. In addition to the seal between the inner and outer wellhead members, another annular seal, or emergency seal, may be located below this seal.
A variety of seals located between the inner and outer wellhead members have been employed in the prior art. FIG. 1 shows a portion of a seal assembly in the prior art within a wellhead housing 10. Housing 10 is typically located at an upper end of a well and serves as an outer wellhead member. An energizing ring 2 is typically forced downward by a running tool or the weight of a string to force it into a slot 3 defined by a U-type metal seal ring 4. This deforms inner and outer walls of the seal ring 4 apart into respective sealing engagement with inner and outer wellhead members 15, 10. The energizing ring is typically a solid wedge-shaped member. The deformation of the inner and outer walls exceeds the yield strength of the material of the seal ring 4, making the deformation permanent. Prior art seals may also include elastomeric and partially metal and elastomeric rings. Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed.
The seals may be set by a running tool, or they may be set in response to the weight of the string of casing or tubing. Located below the seal ring 4 is an emergency seal 5, in case seal ring 4 fails, that rests on a shoulder 6 formed on an inner wellhead member, such as a hanger 15. The emergency seal 5 may be fabricated from metallic, non-metallic, or elastomeric materials, or a combination thereof. The emergency seal 5 may be compressed when downward force from the string is applied to the energizing ring 2 to thereby cause emergency seal 5 to bulge outwards to contact the inner and outer wellhead members 15, 10 at a point below the seal ring 4 above. However, the energizing ring 2 also deforms the metal seal ring 4 against the outer wellhead member 10 and the inner wellhead member 15. If the metal seal ring 4 is deformed against the inner and outer wellhead members 15 and 10 before the emergency seal 5 is compressed sufficiently to bulge outwards against the outer wellhead member 10, then the emergency seal 5 may not be able to perform its function as an emergency seal and pressure integrity may diminish.
A need exists for a technique that addresses the seal leakage problems described above. In particular a need exists for a technique to compress an emergency seal a desired amount prior to deformation of the walls of the metal-to-metal seal. The following technique may solve these problems.